Colorectal cancer (CRC) is the second leading cause of cancer death in the U.S. Approximately 30-50% of CRC patients develop liver metastasis (CRCLM), a major contributor to CRC-related death. As surgical resection of CRCLM becomes increasingly routine, improving outcomes for patients post-CRCLM resection is a high priority. Eicosapentaenoic acid (EPA), a naturally-occurring marine omega-3 polyunsaturated fatty acid may protect against CRC. A recent Phase II randomized placebo-controlled trial (RCT) by our group showed that EPA supplementation improves survival in patients with regional cancer and CRCLM. However, the specific mechanisms through which EPA influences post-operative survival are not well understood. Recent data from our group and others support that the anti-CRC benefit of EPA may be mediated by its pleiotropic roles in modulating the gut microbiota and ameliorating tumor-permissive immunosuppressive mechanisms, including inhibition of the activity of regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), and production of inflammatory mediators such as prostaglandin E2 (PGE2) and chemokine (C-C motif) ligand 2 (CCL2). Dietary fat composition is also a major driver of the gut microbial community structure. Mice fed with a high-EPA diet demonstrate increased abundance of gut bacteria, such as Bifidobacterium and Lactobacillus genera, that support the host immunoprotective system and improve the efficacy of cancer immunotherapy, and decreased abundance of lipopolysaccharide (LPS)-producing bacteria that trigger chronic inflammation and promote CRC. These data together support our hypothesis that the prebiotic effect of EPA abrogates intratumoral immunosuppression and ameliorates systemic inflammation to improve survival of patients with surgical resection of CRCLM. To test this hypothesis, we will leverage our recently launched, phase III RCT of 4-g daily EPA-ethyl ester treatment among 448 patients undergoing liver resection surgery for CRCLM (EPA for Metastasis Trial 2, EMT2), in which participants start treatment at least 2 weeks prior to CRCLM surgery and continue for 2-4 years post-liver resection. Using tissue specimens collected from the post-treatment liver resection, and blood, urine, and stool samples collected at randomization, surgery, and at 6-monthly intervals, we will interrogate immune and microbiome pathways in relation to survival. We will address causality and characterize the mechanisms by which EPA influences the host?microbial interactions to potentiate antitumor immunity and suppress CRCLM in a novel ?avatar? germ-free CRCLM mouse model humanized with stool from RCT participants. Through these integrated investigations, our study may open new avenues for developing EPA-based combinatorial strategies for CRC treatment. The clinical utility of this strategy is particularly appealing due to its cost and safety advantages.